The present invention relates to a diesel particulate filter system used for treatment of exhaust gas from diesel engines. More particularly, the invention relates to an improvement of an air pump system used for feeding air into a burner to burn particulate matter contained in the exhaust of a diesel engine (hereinafter referred to as diesel particulates) trapped in the filter.
Diesel particulates are fine particles primarily composed of hydrocarbons. At high engine speeds, the exhaust gas has a sufficiently high temperature to burn the diesel particulates. However, at low engine speeds, the diesel particulates are discharged into the atmosphere unburnt. In order to prevent air pollution by diesel particulates, a ceramic filter or glass wool filter that traps diesel particulates is provided in the exhaust line. The filter, however, tends to become clogged as a result of prolonged use and requires occasional cleaning, which typically consists of blowing a hot gas onto the filter so as to burn the trapped diesel particulates. For details of an appropriate cleaning technique, reference can be made to Japanese Patent Applications (OPI) Nos. 12029/79 and 131518/80.
A recent achievement in diesel particulate burning systems is a technique for burning diesel particulates with the heat of a flame produced by feeding a fuel, primary air and secondary air, into a burner provided upstream of the diesel particulate filter. Being used for atomizing the fuel, the primary air requires a high pressure but not a high flow rate. On the other hand, the secondary air is used to burn the atomized fuel requires a fairly high flow rate, although it may have a low pressure. In order to meet these requirements, air is conventionally fed to the burner by two moving vane type air pumps of different discharge capacities.